The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel, to maintain the patency thereof. Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the results of the procedure and reduce the possibility of restenosis.
Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use in supporting and holding back a dissected arterial lining which can occlude the fluid passageway there through.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted into a compressed state for deployment into a body lumen. One of the difficulties encountered in using prior art stents involve maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery and accommodate the often tortuous path of the body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from, for example, shape memory metals or super-elastic nickel-titanum (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the body lumen. Such stents manufactured from expandable heat sensitive materials allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent.
Details of prior art expandable stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass, et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No. 5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat. No. 4,655,772 (Wallsten); U.S. Pat. No. 4,739,762 (Palmaz); and U.S. Pat. No. 5,569,295 (Lam), which are hereby incorporated by reference.
Further details of prior art self-expanding stents can be found in U.S. Pat. No. 4,580,568 (Gianturco); and U.S. Pat. No. 4,830,003 (Wolff, et al.), which are hereby incorporated by reference.
Expandable stents are delivered to the target site by delivery systems which often use balloon catheters as the means for expanding the stent in the target area. One such stent delivery system is disclosed in U.S. Pat. No. 5,158,548 to Lau et al. Such a stent delivery system has an expandable stent in a contracted condition placed on an expandable member, such as an inflatable balloon, disposed on the distal portion of an elongated catheter body. A guide wire extends through an inner lumen within the elongated catheter body and out its distal end. A tubular protective sheath is secured by its distal end to the portion of the guide wire which extends out of the distal end of the catheter body and fits over the stent mounted on the expandable member on the distal end of the catheter body.
Some prior art stent delivery systems for implanting self-expanding stents include an inner lumen upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner lumen to xe2x80x9cuncoverxe2x80x9d the compressed stent, allowing the stent to move to its expanded condition into the target area.
The positioning of the stent at the desired location in the body lumen is often critical since inaccurate placement can affect the performance of the stent and the success of the medical procedure. The positioning of the stent before, during, and after its implantation and expansion is generally monitored by external monitoring equipment, such as a fluoroscope, which allows the physician to place the stent in the exact target site. Radiopaque markers placed on the ends of the catheters of the stent delivery system often are utilized to help locate the stent on the catheter during deployment. Additionally, the stent itself can be made from a radiopaque material. For this reason, it is desirable for the stent to be moderately radiopaque. Stents which lack sufficient radiopacity are usually more difficult to position accurately and assess with angiography. Therefore, even a physician using the best available stent delivery systems may not be capable of accurately positioning the stent if there are problems visualizing the stent on the fluoroscope.
Currently, many stents in use are formed from stainless steel or nickel-titanium type alloys which are not always readily visible on the medical imaging instrument. With these materials, the radiopacity of the stent is highly dependant on the amount of metal in the stent. Stents which have thicker struts are generally more radiopaque than stents with thinner struts. However, the strut of the stent cannot be too thick or wide, since the stent must be capable of expanding radially to a larger diameter during deployment. Generally, stents having wider struts are more radiopaque then stents with narrower struts, however, if the strut width is increased in areas subject to high stresses, the strain created in the material at these areas could increase dramatically and may cause cracks in the stent to form, which is highly undesirable.
To increase radiopacity, radiopaque markers have been placed on stents to attempt to provide a more identifiable image for the physician. One such surgical stent featuring radiopaque markers is disclosed in U.S. Pat. No. 5,741,327 (Frantzen) which utilizes radiopaque marker elements attached to the ends of a radially expandable surgical stent to increase the visibility of the stent. However, there are certain drawbacks in utilizing radiopaque markers since some markers may restrict the ability of the stent to fully expand radially and can cause an unwanted protrusion from the surface of the stent which can possibly pierce the wall of the body lumen. Additionally, radiopaque markers can either fail to provide an adequate outline of the stent or illuminate the stent so brightly that fine details such as blood vessels or other bodily structures are obscured when an image is obtained. When the stent is made from highly radiopaque metals such as tantalum or platinum, often the radiopacity of the stent is too high and there is a need to decrease the radiopacity to allow adequate visualization of the surrounding blood vessels, especially at the target location where the PCTA or the PTA procedure has been performed.
The design of the strut pattern of the stent can sometimes lead to unwanted complications during the stent deployment. For example, in some instances, the forces of the body lumen on the fully expanded stent can cause the ends of the stent to collapse somewhat causing the stent to take the form of a xe2x80x9ccigarxe2x80x9d shape, which is highly undesirable. This formation of a xe2x80x9ccigarxe2x80x9d shape is attributable to the fact that the ends of some stents are often not as strong and well supported as the middle section of the stent. Therefore, when the stent is deployed within the body lumen and has a force applied to it, the end portions can compress somewhat before there is any appreciable compression of the middle section of the stent, resulting in the formation of the xe2x80x9ccigarxe2x80x9d shape. This formation of a xe2x80x9ccigarxe2x80x9d shape can lead to abnormal blood flow through the stent, which can cause the formation of unwanted thrombosis, which will be released into the blood stream. A stent design which forms a uniform inner passage way is less likely to cause the formation of thrombosis or blood clots in the bloodstream, and therefore it is highly desirable to have a stent design which has sufficient strength along its entire length to prevent the formation of this unwanted xe2x80x9ccigarxe2x80x9d shape.
What has been needed is a stent which has a high degree of flexibility so it can be advanced through tortuous passageways of the anatomy and can be expanded to its maximum diameter, yet has sufficient strength along its entire length to create a uniform inner-passage for blood flow and provides sufficient radiopacity to present a clear image on a fluoroscope or other medical imaging device during deployment. Such a stent should have sufficient strength and radiopaque properties, yet be configured in a manner which facilitates its manufacture using known manufacturing techniques and utilizes conventional stent delivery systems for implantation in the target location. Moreover, such a stent would be beneficial if it can be manufactured from biocompatible materials which are presently being used in stent designs.
The present invention is directed to stents having strut patterns which enhance the strength of the ends of the stent and the overall radiopacity of the stent, yet retain high longitudinal flexibility along their longitudinal axis to facilitate delivery through tortuous body lumens and remain stable when expanded radially to maintain the patency of a body lumen, such as an artery or other vessel, when implanted therein. The present invention in particular relates to stents with unique end portions having sufficient hoop strength to maintain a constant inner diameter which prevents the stent from taking on a xe2x80x9ccigarxe2x80x9d shape when deployed in the body lumen. The unique end rings of the present invention are particularly useful on self-expanding stents which may otherwise have end rings that may be more susceptible to compression forces. The present invention also relates to the control of the radiopacity of a stent by varying the strut geometry along the stent. By making the width of the strut either wider or narrower in different regions of the stent, the properties of the stent can be customized for a particular application in order to achieve the desired amount of strength and radiopacity for the stent.
Depending upon the strut pattern of the particular stent, there are usually regions or areas of the stent which experience high stresses (referred to as high stress regions) when the strut is expanded radially during deployment. There are other areas or regions of the stent which experience low stresses (referred to as low stress regions) in which stresses on the stent are much lower and any changes in the width of the strut in this region would not dramatically alter the strain and other mechanical properties of the stent. Depending on the material selected for the stent, the width of the stent in the low stress regions can be either increased or reduced in order to change the overall radiopacity of the stent. For example, if a material such as stainless steel or Nitinol is used, the overall radiopacity of the stent can be increased by increasing the width of the strut in the low stress regions. If a high radiopaque material such as tantalum or platinum is used, it is possible to decrease the width of the strut in the low stress regions to decrease the overall radiopacity of the stent, if necessary. In this manner, the addition or decrease of metal in the low stress regions of the strut will effect the overall radiopacity of the stent, yet will not affect the overall strength and mechanical properties of the stent.
Each of the different embodiments of stents of the present invention include a plurality of adjacent cylindrical elements (also known as or referred to as xe2x80x9cringsxe2x80x9d) which are generally expandable in the radial direction and arranged in alignment along a longitudinal stent axis. The cylindrical elements are formed in a variety of serpentine wave patterns transverse to the longitudinal axis and contain a plurality of alternating peaks and valleys. At least one interconnecting member extends between adjacent cylindrical elements and connects them to one another. These interconnecting members insure a minimal longitudinal contraction during radial expansion of the stent in the body vessel. The serpentine patterns have varying degrees of curvature in the regions of peaks and valleys and are adapted so that radial expansion of the cylindrical elements are generally uniform around their circumferences during expansion of the stent from their contracted conditions to their expanded conditions.
Generally, these peaks and valleys of the cylindrical element are subject to high stresses during expansion due to the geometry of the cylindrical element. These valleys and peaks constitute the aptly-named high stress regions of the stent which are susceptible to stress fractures during expansion. For this reason, the width of the strut in the peak and valley portions of the cylindrical element should remain relatively fixed and uniform so that high stresses will not be concentrated in any one particular region of the pattern, but will be more evenly distributed along the peaks and valleys, allowing them to expand uniformly. The regions of the cylindrical element between the peaks and valleys generally form the low stress regions of the stent which do not experience high stresses and strains during radial expansion, thus allowing the width of the stent in these regions to be varied in order to increase or decrease the radiopacity of the stent, as needed, depending upon the material used to manufacture the stent. Generally, these low stress regions extending between the peaks and valleys of the cylindrical element are linear segments which are not subject to high stresses during radial expansion. An increase or decrease of the width of the strut in these low stress regions of the cylindrical element generally will not alter the overall mechanical properties of the stent.
The elongated interconnecting members which connect adjacent cylindrical elements may also have an increase or decrease in strut width in order to change the radiopacity of the stent. In one preferred embodiment, the interconnecting element has a tapered configuration in which one end of the interconnecting member is wider than the other end and tapers from end to end to increase the amount of material, and hence, the radiopacity of the stent. The number and location of the interconnecting members can be varied as may be needed. Generally, the greater the longitudinal flexibility of the stents, the easier and more safely they can be delivered to the implantation site, especially where the implantation site is on a curve section of a body lumen, such as a coronary artery or peripheral blood vessel.
In one particular embodiment of the present invention, the end rings of the stent include peaks and valleys which are made with multiple W or double-curved shapes which enhance both the hoop strength of the stent, along with the radiopacity at the ends of the stent. In this particular embodiment of the invention, the width of the strut in the low-stress regions of the double-curved portion of the cylindrical rings is increased to increase the amount of material at the ends of the stent to enhance radiopacity. This increase in the amount of material at the ends provides satisfactory visibility when one attempts to locate the stent on the fluoroscope. The increase in the mass of the end rings also helps increase the overall strength of the ends of the stent, which helps prevent the stent from collapsing to a xe2x80x9ccigarxe2x80x9d shape during deployment.
The resulting stent structures are a series of radially expandable cylindrical elements that are spaced longitunally close enough so that small dissections in the wall of a body lumen may be pressed back into position against the lumenal wall, but not so close as to compromise the longitudinal flexibility of the stent both when negotiating through the body lumens in their unexpanded state and when expanded into position. Each of the individual cylindrical elements may rotate slightly relate to their adjacent cylindrical elements without significant deformation, cumulatively providing stents which are flexible along their length and about their longitudinal axis, but which still are very stable in their radial direction in order to resist collapse after expansion. An increase or decrease of the strut width in the low stress regions of the stent provides the necessary amount of radiopacity to allow the stent to be adequately visualized on the external monitoring equipment.
The stent of the present invention can be directed to both balloon expandable or self-expanding stent designs. The technique of the present invention can also be applied to virtually any type of stent design. However, it is most easily applied to laser cut stents made from tubing.
The stents of the present invention can be readily delivered to the desired target location by mounting them on an expandable member, such as a balloon, of a delivery catheter and passing the catheter-stent assembly lumen to the target area. A variety of means for securing the stent to the extendible member of the catheter for delivery to the desired location are available. It is presently preferred to crimp or compress the stent onto the unexpanded balloon. Other means to secure the stent to the balloon included providing ridges or collars on the inflatable member to restrain lateral movement, using bioabsorbable temporary adhesives, or adding a retractable sheath to cover the stent during delivery through the body lumen. When a stent of the present invention is made from a self expanding material such as nickel titanium alloy, a suitable stent delivery assembly which includes a retractable sheath, or other means to hold the stent in its expanded condition prior to deployment, can be utilized.
The serpentine pattern of the individual cylindrical elements can optionally be in phase with each other in order to reduce the contraction of the stent along their length when expanded. The cylindrical elements of the stent are plastically deformed when expanded (except with NiTi alloys) so that the stent will remain in the expanded condition and therefore must be sufficiently rigid when expanded to prevent the collapse thereof during use.
In stents formed from super elastic nickel titanium alloys, the expansion occurs when the stress of compression is removed. This allows the phase transformation from martensite back to austenitite to occur, and as a result the stent expands.
One approach to a stent design which relates to the control of stent strength by varying strut geometry along the length of the stent is disclosed in my co-pending application Ser. No. 09/298,063, filed Apr. 22, 1999, by Daniel L. Cox and Timothy A. Limon entitled xe2x80x9cVariable Strength Stent,xe2x80x9d whose contents are hereby incorporated by reference. In that approach, the strength of the stent was enhanced by increasing the length or width of the strut of the cylindrical elements to increase the mass and resulting mechanical strength of the stent. In that approach, certain cylindrical elements having wider struts are located, for instance, in the middle or center section of the stent, while cylindrical elements having narrower strut widths are placed at the ends of the stent to avoid the formation of a xe2x80x9cdog bonexe2x80x9d shape during deployment. However, in this earlier approach, the entire width of the strut in each cylindrical element or ring is maintained uniform throughout, regardless of whether the cylindrical element has a wider or narrower strut pattern. In my present invention described herein, the width of the strut in high and low stress regions of the cylindrical ring is varied accordingly to achieve the desired strength and radiopacity which is needed.
These and other features and advantages of the present invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings.